Current HIV prevention interventions have been predominantly influenced by several cognitive theories. While individual-level theory has been successful in guiding small and intensive group interventions, their effectiveness may be limited in contexts where targeted behaviors are constrained by complex social and environmental factors, especially in developing countries or among communities engaging in illicit behaviors. Community-norm change models that focus on influencing social or peer norms may be more effective in reaching those at highest risk and least likely to volunteer for individually-based interventions. However, there are few studies that evaluated the efficacy of such interventions in developing countries. This study is a randomized controlled trial of a peer educator, network-oriented intervention for injecting drug users (IDUs) and their sexual partners in northern Vietnam. IDUs account for over 88 percent of all reported HIV infections, making them and their drug-using and sexual partners a critical target for HIV prevention efforts. IDU is increasing rapidly in Vietnam, accompanied by an increase in commercial sex work. The potential exists for an explosive shift of transmission dynamics from the core group of IDUs to the general population, as was seen in Thailand. This shift could be mediated by sexual and drug network members as a bridge population to susceptibles in other settings. Current low HIV and STD rates in the general population highlight the need to implement appropriate and effective behavioral interventions in the complex social and ethnographic environment surrounding IDUs and their peer networks. To accomplish this trial, we will: 1) conduct ethnographic research to culturally adapt our peer educator, network oriented intervention and to determine the acceptability of this intervention; 2) enroll and interview a cohort of 400 indexes and three of their sexual/injecting network members (total n = 1600) to determine baseline sexual, drug and biologic (chlamydia, gonorrhea, HBV and HCV) characteristics; 3) randomize index participants to a 6-session network-oriented peer-led intervention or an attention control condition. After randomization, participants will be followed for a minimum of 15 and maximum of 24 months at 6-month intervals. Follow-up visits will consist of a behavioral survey and donation of biological specimens; and 4) compare sexual and drug behavioral risks and HIV VCT uptake among the indexes who receive the intervention and their network members to index IDUs who received the attention controlled sessions and their network members.